An electric power steering (EPS) apparatus is known that applies power of an electric motor to a vehicle steering mechanism to assist steering performed by the driver. For example, Patent Document 1 describes an EPS apparatus including a rack shaft and an electric motor, which are parallel to each other. The EPS apparatus translates rotational motion of the electric motor to linear motion of the rack shaft using a ball screw mechanism to assist steering by the driver.
The EPS apparatus of Patent Document 1 drives the ball screw using a toothed belt attached to the rotary shaft of the electric motor. During linear motion of the rack shaft caused by steering by the driver, for example, one of the rack boots located on opposite ends of the rack shaft expands while the other contracts. Repetitive expansion and contraction of the rack boots may degrade the durability of the rack boots. For this reason, the housing accommodating the rack shaft includes a communication passage, which allows movement of air between the interior space in one of the rack boots and the interior space in the other. This limits expansion and contraction of the rack boots.